The conveying screws for transporting loose material currently present on the market comprise:                a power unit apt to rotate an Archimedes screw pushing the loose material;        a charging hopper of the loose material in a tubular body containing the Archimedes screw and the loose material for at least a section of its feed path;        a feeding channel of the loose material towards a user; the feeding channel is connected in series to said tubular body;        pneumatic pushing elements of the loose material; these pushing elements are present in said feeding channel; and        a back flow preventing device preventing the back flow of pushing air; such a device occupies a transition zone between the tubular body containing the Archimedes screw and the feeding channel.        
This type of conveying screws must be able to continuously charge the loose material falling from the hopper without making the air injected in the form of jet into the feeding channel flow back to the mouth (BC).
Indeed, in the systems currently in use, in order to prevent a portion of the conveying air from reversing the flow and flowing back from the charging mouth of the hopper thus carrying a portion of the loose material outwards, in a manner highly undesirable for users, it has been suggested to exploit the compression of the loose material itself to prevent the releasing of air and powder towards the mouth of the hopper.
In a known solution, a check valve has been adopted to prevent the back-flow of air and powder towards the mouth of the hopper. In this case, a disc is hinged onto a circumferential edge thereof which, due to its weight, closes the through section between the tubular body where the Archimedes screw is contained and the feeding channel.
One of the functional limits of this type of system, however, resides in that for the results to be satisfactory, the powders to be transported must be fine and packable, so that plugs may be formed by reducing the pitch of the Archimedes screw, which plugs follow one another to block the air back flow towards the mouth of the hopper.
Furthermore, transporting abrasive loose products by this system is a further problem because the higher friction with the coils of the Archimedes screw puts at risk the coupling tolerances between the screw itself and the tubular body, and therefore reduces the sealing action exerted by the material present between the coils and the inner wall of the tubular body itself.
In use, once the plug-like material falls into the feeding channel, it is invested by at least one jet of compressed air. Firstly, such a jet breaks up the plug and then impresses the energy needed to feed the powder belonging to the plug towards the end user.